Until other brackets show any sign of having competitive games the organisers of such events won't be willing to hold a tournament in anything other than the 19 bracket.
With that said there's no significant difference within the 19 bracket now compared to 2017, there's a couple new teams, not including LSN who would've played in legion if asked I guarantee it, and in 2017 there were probably a lot of people put off of forming teams when the TC already looked bleak.
It definitely wasn't Conq's theory of toxicity as that didn't change in 2018, it actually got worse with the introduction of endgame players.
It was probably a number of factors that accounted for TC 2017 not happening.
1. Legion being falsely considered a bad expac even though from a wargame standpoint it had so much potential.
2. Conq the organiser having 0 respect from any of the prominent figures in the bracket, if nobody trusts you to run a tournament well then they won't sign up.
3. Due to the rise of private servers, one in particular, there was a distinct lack of plebmades or anything of the like happening, with one or two nights of games happening during the cata servers life, which was roughly 2 and a half months, which tied in with TC announcement.
4. Lack of initial teams and communication, prior to TC 2017 the only teams in existence were GSC and Burg's team NWA who had beat GSC a few months prior, both had signed up to TC, Dan with his usual cryptic sign up that Conq mistook for a fake team and lost faith in them turning up and Burg's team pullin out a few weeks after signing up, much to the anger of everyone, lack of initial interest deterred new teams forming.
TL;DR - 2018 is no different to 2017 it's just 2018 had initial interest that 2017 needed
